Prosthetic heart valves, used to replace diseased natural heart valves, fall generally into two categories. The first category comprises heart valves with relatively rigid leaflets. These valves have one, two or three leaflets formed of a stiff biocompatible substance, such as pyrolitic carbon. The valves are often designed to have two leaflets pivoting in a rigid annulus, such as the design shown in U.S. Pat. No. 4,888,010 to Bokros.
The second category of prosthetic heart valves, called herein bioprosthetic valves, comprises valves with flexible leaflets, frequently of a biological material. This second category can also be divided broadly into two classes. The first class comprises bioprosthetic heart valves typically including a wire frame with three flexible leaflets attached thereto. Examples of such valves are shown in Carpentier, et al., U.S. Pat. No. 4,106,129, Ionescu, et al., U.S. Pat. No. 4,084,268 and Davis, et al., U.S. Pat. No. 4,192,020. These heart valves imitate the natural action of heart valves and so provide a structure which is relatively compatible with the cardiovascular system. However, they are still prosthetic devices, and are subject to wear and fatigue. There is a continuing need to improve the long-term durability of bioprosthetic valves.
Prior valves, such as the valve described in Carpentier, U.S. Pat. No. 4,106,129, comprise wire frames, usually of circular cross-section with three commissures supporting three leaflets. The wire frame could be flexible, but not elastic, because the commissures are relatively rigid. As the valve leaflets move from open to closed positions, bending stresses occur in the portion of the wire frame connecting the commissures. The commissures themselves do not bend significantly. An attempt to overcome the limitations of wire frame valves was proposed by Ross, et al. U.S. Pat. No. 4,343,048. Ross taught that the commissures should be flexible along substantially their entire length, bending in the manner of a fishing pole.
The second class of bioprosthetic heart valves do not have a stent or frame. They have the advantage of being constructed from flexible material, but they can be collapsed and deformed by the action of the heart. The action of the heart muscles on this type of valve can fold the valve material and create unexpected stress risers which can eventually lead to failure.